In an integrated circuit system, when several different levels of reference voltages are needed under various circumstances, a voltage generating circuit is provided therein as shown in FIG. 1. Wherein, a constant current source Ic is used for supplying a constant current, which passes through a plurality of serially connected resistors R1, R2, and R3 . . . Rn. Accordingly, a first reference voltages Vr1, a second reference voltage Vr2, a third reference voltage Vr3, . . . and an Nth reference voltage Vrn are derived from a first node N1, a second node N2, a third node N3, . . . and a Nth node Nn, respectively, followed by forming a plurality of contacts Vo1 . . . Vom via a plurality of multiplexers 11 . . . 1m. The needed reference voltages are then selected by way of controlling the multiplexers.
However, the circuit architecture mentioned above has two disadvantages. First, the design of the multiplexers 11 . . . 1m increases circuit complexity. Moreover, a leakage current of the multiplexers may undesirably influence an accuracy of the output reference voltages. Secondly, several reference voltages are supplied by the plurality of contacts, resulting in complications in the subsequent circuit in which the proper reference voltages are only obtained by switching the contacts. Therefore, it is a primary object of the present invention as to how to overcome the abovementioned disadvantages.